The present invention relates generally to methods for isolating hydrocarbon-containing zones, or production zones intersected by a wellbore. More specifically, the invention relates to a method for sealing the wellbore adjacent zones that have been perforated so that the casing in the wellbore may be perforated and sealed adjacent other zones thereabove.
One operation commonly performed in the completion of an oil or gas well is the perforation of the steel casing of the well to communicate the wellbore with subterranean formations, also referred to herein as production zones, or simply a zone, intersected by the well. Formation fluids are thus allowed to be produced from the formation through the perforations into and up through the wellbore.
Numerous techniques and apparatus are used for conveying perforating guns to perforate the casing, along with associated apparatus into the well including assembling the same on a tubing string thus providing what is commonly referred to as a tubing-conveyed perforating (TCP) system. Perforating guns may also be lowered into a wellbore utilizing coiled tubing and wirelines. Likewise, numerous techniques for actuating perforating guns have been used including (1) electrical actuation; (2) actuation with drop bar mechanisms; and (3) pressure-actuation mechanisms. Perforating guns that are actuated by pressure may be actuated by pressure applied in the tubing or in the casing, depending on the type of perforating gun.
Further, a subterranean formation that has been perforated to provide communication with the wellbore very often must be stimulated or otherwise treated to stimulate the production of fluids. Hydraulic fracturing is a widely used technique for stimulating the production of fluids, such as oil and gas from subterranean formations. Fracturing is commonly performed by contacting a subterranean formation with a viscous fracturing fluid that contains a propping agent suspended therein. The fracturing fluid is communicated with the formation through the perforations. Sufficient hydraulic pressure is applied to the subterranean formation by way of the fracturing fluid and surface pumping equipment to cause one or more fractures to be created in the subterranean formation. After initial fracturing occurs, the fracturing fluid is pumped at a sufficient rate and pressure to cause the fracturing fluid to flow into the created fractures and extend them in the formation. The propping agent suspended in the fracturing fluid is carried into the fracture so that when the flow rate of the fracturing fluid is reduced, the propping agent is deposited into the fractures and the fractures are prevented from closing thereby. Such fracturing increases the permeability of the formation.
Very often a wellbore will intersect more than one subterranean hydrocarbon-containing formation whereby it is desired to perforate the casing adjacent more than one formation and to fracture the formations so that formation fluids, which may also be referred to as production fluids, may be produced up the wellbore from more than one subterranean formation. To perforate the casing adjacent multiple zones and to fracture multiple zones intersected by a single wellbore, zones that have already been communicated with the wellbore by perforating must be isolated while the wellbore is being perforated adjacent other zones. In other words, once the casing has been perforated adjacent a subterranean formation and that formation has been hydraulically fractured, the formation must be isolated prior to perforation of the casing adjacent a formation thereabove and the fracturing of that formation. Methods of perforating and stimulating formations are described in U.S. patent application Ser. No. 08/569,822, filed on Dec. 8, 1995, now U.S. Pat. No. 5,669,448, entitled OVERBALANCE PERFORATING AND STIMULATION METHOD FOR WELLS, assigned to the assignee of the instant application, the details of which are incorporated herein by reference. Techniques employed that allow the perforation and fracturing of multiple zones in a wellbore include the use of retrievable and drillable packers to seal a wellbore so that more than one zone can be perforated and stimulated.
While such presently used techniques are useful, other techniques which are easier to use, less costly and less time-consuming are desired.